Abstract:

A method to produce thin-layer lignocellulosic composites, such as
wood-based doorskins, that exhibit substantial resistance to moisture is
disclosed. In an embodiment, the method includes the steps of forming a
mixture including a refined lignocellulosic fiber, wax, and an organic
isocyanate resin. The mixture is initially pressed to form a loose mat.
Subsequently, the mat is pressed between two dies at an elevated
temperature and pressure to further reduce the thickness of the mat and
to promote the interaction of the resin with the lignocellulosic fibers.
In an embodiment, a release agent is included as part of the fiber
mixture, or sprayed onto the surface of the mat. The thin-layer
lignocellulosic composites of the present invention exhibit strong
surface strength, high adhesiveness, and a 50% reduction in linear
expansion and thickness swelling upon exposure to a high moisture
environment as compared to thin-layer composites that do not include the
isocyanate resin.

Claims:

1. A thin-layer lignocellulosic composite comprising a mixture of no more
than 95% by weight of at least one type of lignocellulosic fiber, wherein
the fiber has a predetermined moisture content, at least 5% by weight of
an organic isocyanate resin, a release agent that does not interfere with
processing of the thin-layer lignocellulosic composite, and at least one
type of wax, wherein the mixture is pressed between two dies at an
elevated temperature and pressure and for a sufficient time to form a
thin-layer composite of predetermined thickness, and to allow the
isocyanate resin to interact with the lignocellulosic fiber such that the
resultant thin-layer composite has a predetermined resistance to
moisture.

5. The thin-layer lignocellulosic composite of claim 1, wherein the
release agent comprises an emulsion of surfactants and polymers.

6. The thin-layer lignocellulosic composite of claim 1, wherein the
release agent is added to the wood mixture prior to pressing the mixture
into a thin-layer composite.

7. The thin-layer lignocellulosic composite of claim 6, wherein the amount
of release agent added to the composite ranges from about 0.5% to about
8% by weight.

8. The thin-layer lignocellulosic composite of claim 1, wherein the
mixture is preformed into a loose mat, and the release agent is sprayed
onto at least one surface of the mat prior to pressing the mixture into a
thin layer composite.

9. The thin-layer lignocellulosic composite of claim 8, wherein the amount
of release agent sprayed on to the mat surface comprises 0.1 to 8.0 grams
solids per square foot (1.1 to 86.1 grams per square meter) of the
surface.

18. The thin-layer lignocellulosic composite of claim 1, wherein the
predetermined resistance to moisture comprises up to a 50% reduction in
linear expansion and thickness swelling after being immersed for 24 hours
in 70.degree. F. (21.degree. C.) water than a thin-layer composite
comprising a resin that does not include isocyanate.

19. The thin-layer lignocellulosic composite of claim 1, wherein said
predetermined resistance to moisture comprises a thickness swelling of
less than 15% after being immersed for 24 hours in water at 70.degree. F.
(21.degree. C.).

[0002]A section of the disclosure of this patent document and its figures
contain material subject to copyright protection. The copyright owner has
no objection to the facsimile reproduction by anyone of the patent
document, but otherwise reserves all copyright rights whatsoever.

FIELD OF THE INVENTION

[0003]The present invention relates to the manufacture of thin-layer
lignocellulosic composites, such as wood-based doorskins. More
particularly, the present invention relates to thin-layer wood composites
that contain an isocyanate based-resin and thus, exhibit significantly
less swelling and/or shrinking upon exposure to the environment.

BACKGROUND OF THE INVENTION

[0004]A significant problem in the manufacture of wood-based composite
products that are exposed to the exterior and extreme interior
environments is that upon exposure to variations in temperature and
moisture, the wood can lose water and shrink, or gain water and swell.
This tendency to shrink and/or swell can significantly limit the useful
lifetime of most exterior wood products, such as wooden doors, often
necessitating replacement after only a few years. The problem is
particularly prevalent in areas of high moisture (e.g., Hawaii) or in
climates that are extremely hot or dry (e.g., Arizona). Shrinking and
swelling can also be a problem when the wood is exposed to a wet
environment during construction, or upon exposure to the dry heat used
indoors in the winter.

[0005]A possible solution to the problem of moisture gain and loss in wood
exposed to the elements includes covering the wood with paint and/or
other coatings that act as a barrier to moisture. Still, such coatings
tend to wear off with time, leaving the wood susceptible to the
environment.

[0006]Rather than treating the unit at the site of installation, it may be
preferable to manufacture products that exhibit increased resistance to
moisture gain and loss. For example, increasing the amounts of resin
content or decreasing the amount of wood fiber used in a door can
increase resistance to water gain and water loss. However, such
modifications can be associated with significantly increased production
costs. Other options include the use of metal or fiberglass doors, but
such doors are not always as aesthetically pleasing as wood doors and may
have other performance problems associated with the use of these
materials.

[0007]Alternatively, doors, and other structural units, may be covered
with a wood-containing water-resistant layer. For example, doors may be
covered with a thin-layer wood composite known as a doorskin. Doorskins
are molded as thin layers to be adhesively secured to an underlying door
frame to thereby provide a water-resistant outer surface. Doorskins may
be made by mixing wood fiber, wax, and a resin binder, and then pressing
the mixture under conditions of elevated temperature and pressure to form
a thin-layer wood composite that is then bonded to the underlying door
frame.

[0008]Wood composite doorskins are traditionally formed by pressing wood
fragments in the presence of a binder at temperatures exceeding
275° F. (135° C.). The resin binder used in the doorskin
may be a formaldehyde-based resin, an isocyanate-based resin, or other
thermoplastic or thermoset resins. Formaldehyde-based resins typically
used to make wood composite products include phenol-formaldehyde,
urea-formaldehyde, or melamine-formaldehyde resins. Phenol-formaldehyde
resins require a high temperature cure and are sensitive to the amount of
water in the wood since excess water can inhibit the high temperature
cure. Urea and melamine-formaldehyde resins do not require as high of a
temperature cure, but traditionally do not provide comparable
water-resistance (at the same resin content) in the doorskin product.

[0009]As compared to doorskins made using phenol-formaldehyde resins,
doorskins that utilize high-temperature pressed isocyanate resin binder
display increased surface strength. However, these doorskins exhibit
decreased porosity to adhesives and thus, do not bond well to the
underlying doorframe. Also, isocyanate-bonded wood composites made using
currently available methods and compositions do not consistently exhibit
sufficient resistance to environmentally-induced swelling and/or
shrinking to be commercially useful. Thus, there remains a need for a
commercially viable method to produce a thin-layer wood composite that
displays resistance to shrinking and swelling. Such thin-layer wood
composites are useful to protect doors and other wood-based structures
exposed to the environment.

SUMMARY

[0010]Embodiments of the present invention comprise thin-layer
lignocellulose composites having increased resistance to moisture and
methods of making the same. An example embodiment of the present
invention comprises thin-layer lignocellulosic composites that are
formulated using an isocyanate resin and thus, exhibit significantly less
swelling and/or shrinking upon exposure to the environment. In an
embodiment, the present invention comprises a thin-layer lignocellulosic
composite comprising no more than 95% by weight of a lignocellulosic
fiber and at least 5% by weight of an organic isocyanate resin. In an
embodiment, the lignocellulosic fiber comprises refined wood fiber. The
lignocellulosic composite may further include wax. Also, the composite
may include a release agent, wherein the release agent is added directly
to the composite, and/or is sprayed onto the surface of the composite
product. Also, the fiber used to make the composite may comprise a
predetermined moisture content. Generally, the moisture content of the
fiber is such that a dehydration step is not required to cure with the
isocyanate resin. The thin-layer lignocellulosic composites of the
present invention exhibit strong surface strength, high bonding
capabilities, and up to a 50% reduction in linear expansion and thickness
swelling upon exposure to a high moisture environment as compared to
thin-layer composites that are made using other (non-isocyanate) resins.

[0011]Embodiments of the present invention also comprise methods for
making thin layer lignocellulosic composites having high moisture
resistance. In an embodiment, the method includes forming a mixture
comprising a refined lignocellulosic fiber comprising a predefined
moisture content and at least 5% by weight of an organic isocyanate resin
and pre-pressing the mixture into a loose mat. Subsequently, the mat is
pressed between two dies at an elevated temperature and pressure to
further reduce the thickness of the mat and to promote the interaction of
the resin with the lignocellulosic fibers. In an embodiment, the fibers
are wood fibers. Also, in an embodiment, a release agent is included as
part of the mixture, and/or is sprayed onto the surface of the mat.
Additionally and/or alternatively, wax may be added to the
lignocellulosic composite mixture.

[0012]From the foregoing summary, it is apparent that an object of the
present invention is to provide methods and compositions relating to the
production of wood products that are resistant to the environment. It is
to be understood that the invention is not limited in its application to
the specific details as set forth in the following description, figures
and claims. The invention is capable of other embodiments and of being
practiced or carried out in various ways.

BRIEF DESCRIPTION OF THE FIGURES

[0013]FIG. 1 shows an embodiment of a method that may be used to make a
thin-layer wood composite doorskin.

[0014]FIG. 2 shows an embodiment of a method used to make water-resistant
thin-layer wood composites in accordance with an embodiment of the
present invention where panel (a) shows mixing of the lignocellulosic
fiber and resin; panel (b) shows forming the composite into a loose mat;
panel (c) shows spraying the loose mat with release agent; panel (d)
shows pressing the mat between two dies; and panel (e) shows the
resultant thin-layered composite product.

DETAILED DESCRIPTION

[0015]The present invention provides for the manufacture of thin-layer
lignocellulosic composites that include levels of isocyanate-based resins
that protect the composite from shrinking and swelling upon exposure to
the elements. The invention may be applied to various types of
lignocellulosic thin-layer composites to generate structural units that
may be exposed to weathering by heat, moisture, air, and the like. In an
embodiment, the

[0016]present invention describes a method to make wood-based doorskins
that are resistant to shrinking and swelling.

[0017]Thus, in an embodiment, the present invention comprises a method to
produce a thin-layer lignocellulosic composite having increased
resistance to moisture-induced shrinking and swelling comprising the
steps of: (a) forming a lignocellulosic composite mixture comprising at
least one type of lignocellulosic fiber comprising a predefined moisture
content and at least 5% by weight of an organic isocyanate resin; (b)
pre-pressing the mixture into a loose mat; and (c) pressing the mat
between two dies at an elevated temperature and pressure and for a
sufficient time to further reduce the thickness of the mat to form a
thin-layer composite of predetermined thickness, and to allow the
isocyanate resin to interact with the lignocellulosic fiber such that the
resultant thin-layer composite has a predetermined resistance to
moisture.

[0018]The present invention also comprises thin-layer lignocellulosic
composites made by the methods of the invention. Thus, in another
embodiment, the present invention also comprises a thin-layer
lignocellulosic composite comprising a mixture of no more than 95% by
weight of at least one type of lignocellulosic fiber, wherein the fiber
has a predetermined moisture content, and at least 5% by weight of an
organic isocyanate resin, wherein mixture is pressed between two dies at
an elevated temperature and pressure and for a sufficient time to form a
thin-layer composite of predetermined thickness, and to allow the
isocyanate resin to interact with the lignocellulosic fiber such that the
resultant thin-layer composite has a predetermined resistance to
moisture.

[0019]The lignocellulosic fiber comprises a material containing both
cellulose and lignin. Suitable lignocellulosic materials may include wood
particles, wood fibers, straw, hemp, sisal, cotton stalk, wheat, bamboo,
jute, salt water reeds, palm fronds, flax, groundnut shells, hard woods,
or soft woods, as well as fiberboards such as high density fiberboard,
medium density fiberboard, oriented strand board and particle board (see
e.g., U.S. Pat. No. 6,620,459 for a description of lignocellulosic
fibers). In an embodiment, the lignocellulosic fiber is refined. As used
herein, refined fiber comprises wood fibers and fiber bundles that have
been reduced in size from other forms of wood such as chips and shavings.
The refined wood fiber is normally produced by softening the larger wood
particles with steam and pressure and then mechanically grinding the wood
in a refiner to produce the desired fiber size. In an embodiment, the
lignocellulosic fiber of the thin-layer composites of the present
invention comprise wood fiber.

[0020]As used herein, a thin-layer composite comprises a flat, planar
structure that is significantly longer and wide than it is thick.
Examples of thin-layer lignocellulosic composites include wood-based
doorskins that are used to cover the frame of a door to provide the outer
surface of the door. Such doorskins may be only about 1 to 5 mm thick,
but may have a surface area of about 20 square feet (1.86 square meters)
or more. Other thin-layer lignocellulosic products may include Medium
Density Fiberboard (MDF), hardboard, particleboard, Oriented Strand Board
(OSB) and other panel products made with wood. These products are
normally 3 to 20 mm in thickness.

[0021]In an embodiment, the lignocellulosic composite mixture further
comprises at least one type of wax. For example, the mixture may comprise
up to about 2% by weight of wax. In an embodiment, about 0.5% by weight
wax is used.

[0022]The wax may impart additional short-term water repellency to the
wood composite. The type of wax used is not particularly limited, and
waxes standard in the art of wood fiber processing may be used.
Generally, the wax should be stable to the temperatures used for pressing
the wood/resin mixture into a thin layer, increase the water repellency
of the wood, and not adversely affect the aesthetics or subsequent
processing (such as priming or gluing) of the wood composite. Thus, the
wax may be a natural wax or a synthetic wax, generally having a melting
point in the range of about 120° F. (49° C.) to about
180° F. (82° C.). Waxes used may include, but are not
limited to, paraffin wax, polyethylene wax, polyoxyethylene wax,
microcrystalline wax, shellac wax, ozokerite wax, montan wax, emulsified
wax, slack wax, and combinations thereof.

[0023]As described herein, the lignocellulosic mixtures of the present
invention are pressed into thin-layers using flat or molded dies at
conditions of elevated temperature and pressure. In an embodiment, the
mixture is initially formed into a loose mat, and the mat is placed in
the die press. Because the composite includes amounts of resin that are
sufficient to increase the water resistance of the composite mixture, the
composite may stick to the surface of the dies that are used to press the
mat into the resultant thin layer composite. Thus, in an embodiment, the
method includes steps to reduce sticking of the thin-layer composite to
the dies.

[0024]In an embodiment, the method includes exposing the lignocellulosic
composite mixture to a release agent prior to pressing the composite
between the dies. In an embodiment, the release agent comprises an
aqueous emulsion of surfactants and polymers. For example, the release
agent may comprise compounds used in the doorskin manufacturing industry
such as, but not limited to, PAT®7299/D2 or PAT® 1667 (Wurtz GmbH
& Co., Germany).

[0025]The release agent may be added directly to the lignocellulosic
composite mixture as an internal release agent prior to pre-pressing the
mixture into a loose mat. Alternatively and/or additionally, the release
agent may be sprayed on the surface of the mat before the mat is pressed
into a thin layer.

[0026]Where the release agent is added directly to the mixture as an
internal release agent, the amount of release agent added may range from
about 0.5 to about 8 weight percent of the mixture. In one embodiment,
about 2 weight percent release agent is used.

[0027]Where the release agent is sprayed onto a surface of the mat, the
amount of release agent sprayed on to the mat surface may comprise from
about 0.1 to about 8.0 grams solids per square foot (1.1 to 86.1 grams
per square meter) of mat surface. In another embodiment, the amount of
release agent sprayed on the mat surface may comprise about 4 grams
solids per square foot (43 grams per square meter) of mat surface. The
release agent may be applied as an aqueous solution. In an embodiment, an
aqueous solution of about 25% release agent is applied to the mat
surface. When the thin-layer composite comprises a doorskin, the release
agent may be applied to the surface of the mat that corresponds to the
surface that will become the outer surface of the doorskin.

[0028]In an embodiment, the thin-layered lignocellulosic composite is
colored. For example, in one embodiment, the release agent may comprise a
pigment. In this way, an even coloring is applied to the thin-layered
lignocellulosic composite.

[0029]Thus, the thin-layer lignocellulosic composites of the present
invention may comprise wood fibers as well as wax and/or a release agent.
For example, in an embodiment, the present invention comprises a wood
composite comprising a mixture of: (i) no more than 95% by weight of a
wood fiber, wherein the wood fiber has a predetermined moisture content;
(ii) at least 5% by weight of an organic isocyanate resin; (iii)
optionally, at least 0.5% by weight of a wax; and (iv) optionally, at
least 1% internal release agent by weight and/or at least 0.1 grams
release agent per square foot (1.1 grams per square meter) on the surface
of the composite.

[0030]Other strategies may be used to reduce sticking of the
lignocellulosic composite to the dies used for making the resultant
thin-layer composite. Thus, in another embodiment, at least one surface
of the die used to press the mat is exposed to an anti-bonding agent. In
an embodiment, exposing the die to an anti-bonding agent may comprise
coating at least one of the dies used to press the mat with an
anti-bonding agent. In an embodiment, coating the die may comprise baking
the anti-bonding agent onto the die surface.

[0031]In an embodiment, the release agent is not the same as an
anti-bonding agent. The release agent comprises a compound that will not
interfere with subsequent processing of the resulting thin-layer
composite. In contrast, the anti-bonding agent may comprise compositions
known in the art of pressing wood composites as being effective in
preventing sticking to the pressing dies, but that may be problematic if
included as part of the composite.

[0032]For example, in an embodiment, the anti-bonding agent used to coat
the die surface comprises silane or silicone. Thus, the anti-bonding
agent used to coat the die surface may comprise anti-bonding agents known
in the art of die pressing such as, but not limited to, CrystalCoat MP-3
13 and Silvue Coating (SDC Coatings, Anaheim, Calif.), Iso-Strip-23
Release Coating (ICI Polyurethanes, West Deptford, N.J.),
aminoethylaminopropyltrimethoxysilane (Dow Corning Corporation), or the
like.

[0033]For thin-layer doorskins, the die that is coated with the
anti-bonding agent may preferably correspond to the die used to press the
outside surface of the doorskin. Alternatively, both dies may be coated
with an anti-bonding agent. In an embodiment, the amount of anti-bonding
agent used to coat the die surface may range in thickness from about
0.0005 to about 0.010 inches (i.e., about 0.0127 mm to about 0.254 mm).
Thus, in one embodiment, the amount of anti-bonding agent used to coat
the die surface comprises about 0.003 inches (i.e., about 0.0762 mm).

[0034]In an embodiment, coating the die comprises baking the anti-bonding
agent onto the die surface. For example, in one embodiment, the step of
baking the anti-bonding agent onto the die surface may comprise the steps
of: (1) cleaning the die surface free of dirt, dust and grease; (ii)
spraying from about 0.0005 to 0.010 inches (0.5 to 10 mils or about
0.0127 to 0.254 mm) of a 50% solution of the anti-bonding agent onto the
die; and (iii) baking the die at greater than 300° F. (149°
C.) for about 1 to 4 hours.

[0035]In an embodiment, the step of exposing the pre-pressed mat to at
least one release agent and/or anti-bonding agent may comprise adding an
internal release agent and/or spraying one side of the mat with a release
agent and also coating at least one die surface with an anti-bonding
agent. In this embodiment, the side of the mat coated with the release
agent is the surface opposite to the surface of the mat exposed to the
coated die. For example, in an embodiment, the present invention
comprises a method to produce a thin-layer wood composite having
increased water resistance comprising the steps of: (a) forming a mixture
comprising: (i) a refined wood fiber comprising a predefined moisture
content; (ii) a wax; (iii) at least 5% by weight of an organic isocyanate
resin; and (iv) optionally, a release agent; (b) pre-pressing the mixture
into a loose mat; (c) optionally, spraying one surface of the mat with a
release agent; and (d) pressing the mat between two dies at an elevated
temperature and pressure and for a sufficient time to further reduce the
thickness of the mat to form a thin-layer composite of predetermined
thickness, and to allow the isocyanate resin to interact with the wood
fibers such that the doorskin has a predetermined resistance to moisture,
wherein at least one of the die surfaces has been coated with an
anti-bonding agent.

[0036]The thin-layered lignocellulosic composites of the present invention
may comprise a range of fiber compositions. Thus, in an embodiment, the
lignocellulosic composite mixture comprises about 80% to about 95% by
weight fiber.

[0037]The thin-layered wood composites of the present invention may
comprise lignocellulosic fiber comprising a range of moisture levels. In
an embodiment, the method does not require dehydrating the
lignocellulosic fiber prior to treatment with the resin. Thus, in an
embodiment, the lignocellulosic fiber comprises from about 7% to about
20% moisture content by weight. In another embodiment, the
lignocellulosic fiber may comprise from about 10% to about 14% moisture
by weight.

[0038]The organic isocyanate resin used may be aliphatic, cycloaliphatic,
or aromatic, or a combination thereof. Also, although monomers may be
preferred, polymeric isocyanates may also be used. In an embodiment, the
isocyanate may comprise diphenylmethane diisocyanate (MDI) or toluene
diisocyanate (TDI) such as Lupranate® M2OFB Isocyanate (BASF
Corporation, Wyandotte, Mich.). For example, in an embodiment, the
isocyanate comprises diphenylmethane-4,4'-diisocyanate. Or, in an
embodiment, the isocyanate is selected from the group consisting of
toluene-2,4-diisocyanate; toluene-2,6-diisocyanate; isophorone
diisocyanate; diphenylmethane-4,4'-diisocyanate;
3,3'-dimethyldiphenylmethane-4,4'-diisocyanate; m-phenylene diisocyanate;
p-phenylene diisocyanate; chlorophenylene diisocyanate;
toluene-2,4,6-triisocyanate; 4,4',4''-triphenylmethane triisocyanate;
diphenyl ether 2,4,4'-triisocyanate; hexamethylene-1,6-diisocyanate;
tetramethylene-1,4-diisocyanate, cyclohexane-1,4-diisocyanate;
naphthalene-1,5-diisocyanate; 1-methoxyphenyl-2,4-diisocyanate;
4,4'-biphenylene diisocyanate; 3,3'-dimethoxy-4,4'-biphenyl diisocyanate;
3,3'-dimethyl-4,4'-biphenyl diisocyanate;
4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate;
3,3'-dichlorophenyl-4,4'-diisocyanate;
2,2',5,5'-tetrachlorodiphenyl-4,4'-diisocyanate; trimethylhexamethylene
diisocyanate; m-xylene diisocyanate; polymethylene polyphenylisocyanates;
and mixtures thereof (see e.g., U.S. Pat. No. 5,344,484 for a description
of isocyanates that may be used to formulate wood doorskins).

[0039]A range of isocyanate resin levels may be used to make the
thin-layer composites of the present invention. Thus, in an embodiment,
the mixture used to form the 20 composite may comprise from about 6.5% to
about 15% by weight resin solids. In another embodiment, the mixture may
comprise about 10% by weight resin solids.

[0040]The conditions used to form the thin-layer composite include
compressing the mixture at elevated temperature and pressure for
sufficient time to allow the isocyanate resin to interact with the wood
fibers such that the resultant thin-layer composite has a predetermined
resistance to moisture. The exact conditions used will depend upon the
equipment used, the exterior environment (e.g., temperature, elevation),
the manufacturing schedule, the cost of input resources (e.g., starting
materials, electric power), and the like. Also, varying the temperature
may allow for changes to be made in the pressure used or the time of
pressing; similarly, changes in pressure may require adjustment of the
time and/or temperature used for pressing the thin-layer composites of
the present invention.

[0041]A range of temperatures may be used to promote interaction of the
isocyante resin with the lignocellulosic fibers in the mixture. In an
embodiment, the temperature used to press the mixture (or preformed mat)
into a thin-layer composite may range from about 250° F.
(121° C.) to about 400° F. (204° C.). In another
embodiment, the temperature used to press the mixture (or preformed mat)
into a thin-layer composite may range from about 280° F.
(138° C.) to about 350° F. (177° C.). Or, a
temperature that is in the range of from about 310° F.
(154° C.) to about 330° F. (166° C.) may be used.

[0042]Similarly, the levels of the pressure applied during the pressing of
the thin-layer composite may vary depending on a variety of factors, such
as the nature of the thin-layer composite that is being formed, the
equipment being used, environmental conditions, production capabilities,
and the like. Thus, in an embodiment, the pressure during the pressing
step may range from about 2500 psi (176 kg/cm2) to about 150 psi
(10.5 kg/cm2). In another embodiment, the pressure may be applied in
a step-wise manner. In another embodiment, the pressure during the
pressing step ranges from about 1200 psi (84.3 kg/cm2) for about 5
to 20 seconds followed by 500 psi (35.16 kg/cm2) for 20 to 80
seconds. For example, in one embodiment, the pressure during the pressure
step ranges from about 1200 psi (84.3 kg/cm2) for about 10 seconds
to about 500 psi (35.16 kg/cm2) for about 50 seconds.

[0043]The thin-layer lignocellulosic composites of the present invention
have increased resistance to moisture-induced shrinkage and swelling. As
used herein, increased resistance to moisture comprises reduced shrinking
and/or swelling of the thin-layer composite when the composite is exposed
to conditions of low and high moisture, respectively, as compared to thin
lignocellulosic composites made by other methods, or using non-isocyanate
resins. As used herein, a normal moisture level of a thin-layer composite
typically ranges between 6% and 9%. Moisture contents below this range
may be considered low moisture, and moisture contents above this range
may be considered high moisture.

[0044]Thus, in an embodiment, when thin-layer composites of the present
invention are exposed to an atmosphere where the moisture level is low,
the composite of the present invention exhibits less shrinkage than
thin-layer composites made with other resins. Also, in an embodiment,
when thin-layer composites of the present invention are exposed to an
atmosphere where the moisture level is high, the composite of the present
invention exhibits less swelling than thin-layer composites made with
other resins.

[0045]For example, in an embodiment, the thin-layer composite comprises up
to 50% less linear expansion and thickness swelling after being immersed
for 24 hours in 70° F. (21° C.) water than a thin-layer
composite comprising comparable levels of an alternate (non-isocyanate)
resin, or lower amounts of the isocyanate resin. Also in an embodiment,
the predetermined resistance to moisture comprises a thickness swelling
of less than 15% after being immersed for 24 hours in water at 70°
F. (21° C.).

[0046]Also in an embodiment, doorskins made by the methods of the present
invention are significantly less dense than doorskins made using
traditional formaldehyde-based resins. Thus, in an embodiment, the
thin-layer lignocellulosic composites of the present invention comprise a
density of less than 60 pounds per cubic foot (962 kg/m3). In
another embodiment, the thin-layer lignocellulosic composites of the
present invention may comprise a density of less than 55 pounds per cubic
foot (881.5 kg/m3).

Preparation of Thin-Layer Wood Composites Having Increased Water
Resistance

[0047]Several methods have been explored to produce wood composites that
exhibit increased resistance to moisture uptake and loss. It is believed
that swelling and/or shrinking of wood is, at least partially, the result
of water reacting with hydroxyl groups present in cellulose and
hemicellulose. Thus, high moisture levels increase the amount of water
bound to the wood fiber. Alternatively, in low humidity, water is lost
from the wood fibers.

[0048]Wood may be treated with chemical agents to modify the hydroxyl
groups present in the cellulose and to thereby reduce the reactivity of
cellulose fibers with water. For example, acetylation of cellulose fibers
can reduce the number of hydroxyl groups available to react with water
and thus, makes the wood less susceptible to heat-induced drying or
moisture-induced swelling. Still, on a large scale, acetylation may not
be commercially viable as it is expensive to perform and entails
significant disposal costs.

[0049]Formaldehyde resins may also be used as a means to modify the
hydroxyl groups in cellulose fibers as a result of the formaldehyde
bonding to the hydroxyl sites in cellulose. For example,
phenol-formaldehyde resins may be used. However, the phenol-formaldehyde
resins require high temperature and pressure for curing. Such resins
cannot be used efficiently with wood that has a moisture content of
greater than 8% as the water interferes with the curing step. Thus, use
of phenol-formaldehyde resins requires drying the wood prior to curing.
After curing, the wood must then be re-hydrated to increase the moisture
level of the wood such that a wood composite having acceptable commercial
properties is achieved.

[0050]Alternatively, fibers from non-wood sources that may have reduced
cellulose can be employed, such as fiber from corn and flax seed. Still,
these fibers are not typically used to make composites because these
fibers are often not consistently available or as economical as wood
fiber.

[0051]The present invention is concerned with methods to employ isocyanate
resins to improve the moisture-resistance of thin-layer lignocellulosic
composites, such as, but not limited to, wood doorskins. Isocyanate
resins such as diphenylmethane-4,4'-diisocyanate (MDI) and toluene
diisocyanate (TDI) resin are highly effective in modifying the reactive
groups present on cellulose fibers to thereby prevent the fibers from
reacting with water. It is believed that the isocyanate forms a chemical
bond between the hydroxyl groups of the wood cellulose, thus forming a
urethane linkage.

[0052]Efforts to develop isocyanate resins for thin-layer wood composites
are described in U.S. Pat. No. 3,440,189, describing the use of
isocyanate resin and a basic catalyst, U.S. Pat. No. 4,100,138,
describing the use of an isocyanate and a polyether polyol binder, as
well as U.S. Pat. No. 4,359,507, describing use of isocyanates mixed with
ethylene carbonate and propylene carbonate as a binder. Also, U.S. Pat.
No. 6,620,459 describes a method for impregnating wood substrates with an
isocyanate resin by dipping the wood in the resin followed by subsequent
polymerization steps, and U.S. Pat. Nos. 4,388,138 and 4,396,673 describe
use of a binder of polyisocyante and a wax release agent. U.S. Pat. No.
5,344,484 describes the use of low-temperature pressing to prepare
isocyanate-bonded wood composites described as having high surface
strength but porous enough such that adhesives can bond the treated
thin-layer composite to an underlying wood frame. U.S. Pat. No. 5,344,484
describes that such wood composites include 1 to 4% isocyanate resin.
Still, it has been found that such low levels of resin that do not
provide consistent levels of moisture resistance to thin-layer wood
composites.

[0053]To provide a thin-layer wood composite that is resistant to water,
resin contents of greater than 5%, and more preferably at levels of about
10%, up to about 15%, are required. However, there are problems when
manufacturing thin-layer lignocellulosic composites using
isocyanate-based resins at concentrations greater than 5%. For example,
doorskins are generally on the order of 2 to 5 mm in thickness, with a
total surface area of 20 square feet (i.e., 1.86 square meters). When
such thin-layer wood composites made with 10% isocyanate resin are
prepared using conventional pressing methods, the high resin levels cause
the wood composite to stick to the pressing die used to prepare the
doorskin after only a few pressing cycles.

[0054]FIG. 1 shows an overview of a general method used to prepare
doorskins. Generally, a selected wood starting material is ground to
prepare fibers of a uniform size and the appropriate amount of wax added.
At this point the preparation may be stored until further processing. The
fiber/wax blend is then mixed with an appropriate binder resin (e.g.,
using atomization), until a uniform mixture is formed. It is also common
to add the resin to the fiber prior to storage of the fiber.

[0055]The mixture may then be formed into a loose mat which is pre-shaped
using a shave-off roller and pre-compressed to a density of about 6-8
pounds per cubic foot. After further trimming to the correct size and
shape, the pre-pressed mat is introduced into a platen press, and
compressed between two dies under conditions of increased temperature and
pressure. For example, standard pressing conditions may comprise pressing
at 320° F. at 1200 psi for 10 seconds followed by 50 seconds at
500 psi (i.e., about 160° C. at 84.3 kg/cm2 for 10 seconds
followed by 50 seconds at 35.2 kg/cm2). Generally, a recessed
(female) die is used to produce the inner surface of the doorskin, and a
male die shaped as the mirror image of the female die is used to produce
the outside surface of the skin. Also, the die which is forming the side
of the doorskin that will be the outer surface may include an impression
to create a wood grain pattern. After cooling, the resulting doorskin is
mounted onto a doorframe using a standard adhesive and employing methods
standard in the art.

[0056]Embodiments of the present invention recognize that the use of a
release agent and/or an anti-bonding agent during the manufacture of wood
composite doorskins allows for increased levels of resin to be used for
the manufacture of doorskins made by low-temperature pressing.

[0057]Thus, in an embodiment (FIG. 2), the present invention describes a
method for making a thin-layer wood composite having increased water
resistance comprising forming a wood composite mixture 2 comprising: (i)
a refined wood fiber 4 having a predefined moisture content of about 10
to 14%; (ii) 0.5 to 2.0% wax; (iii) greater than 5% by weight of an
organic isocyanate resin; and (iv) optionally, at least 1% by weight of
an internal release agent (FIG. 2(a)). The mixture may be prepared in
bulk using standard blowline blending of the resin and fibers. Or,
blenders 9 having a means for mixing 3 such as a paddle or the like, may
be used.

[0058]Next, the wood composite mixture may be formed into a loose mat in a
forming box. The mat is then pre-shaped using a shave-off roller (not
shown in FIG. 2) and precompressed using a roller or some other type of
press 7 (FIG. 2(b)). The specific density of the mat may vary depending
on the nature of the wood composite being formed, but generally, the mat
is formed to have a density of about 6 to 8 pounds per cubic foot (i.e.,
96.2-128.1 kg per cubic meter). After further trimming of the mat to the
correct size and shape, at least one surface of the mat may be exposed to
additional release agent 8 by spraying the release agent onto the surface
of the mat 6 using a spray nozzle 11 (FIG. 2(c)). Also, shown in FIG. 2
are conveyors 5 and 13 as a means for transferring the wood composite
from one station to another. It is understood that other means of
supporting or transferring the thin-layer wood composite from one station
to another, or supporting the composite during the processing steps may
be used.

[0059]The mat 6 may then be placed between a male die 14 and a female die
12, and pressed at an elevated temperature and pressure and for a
sufficient time to further reduce the thickness of the thin-layer
composite and to allow the isocyanate resin to interact with the wood
fibers (FIG. 2(d)). As described above, it is believed that by heating
the wood composite in the presence of the resin, the isocyanate of the
resin forms a urethane linkage with the hydroxyl groups of the wood
cellulose. Replacement of the hydroxyl groups of the cellulose with the
urethane linkage prevents water from hydrating or being lost from with
the cellulose hydroxyl groups. Thus, once the resin has cured, a doorskin
having a predetermined resistance to moisture is formed. As described
above, in an embodiment, one of the dies may be coated with an
anti-bonding agent. FIG. 2 shows and embodiment in which the female die
12 is coated on its inner surface with an anti-bonding agent 10.

[0060]In alternative embodiments, both dies (12 and 14) are coated with
anti-bonding agent. For example, this embodiment may be preferred where
both die surfaces do not have a grain pattern, but are smooth. Or, in an
embodiment, both inner die surfaces may be coated with an anti-bonding
agent, and the use of release agent to coat the mat may vary depending
upon the particular wood composite being prepared. Or, in an embodiment,
the method may employ release agent on the surface of the mat, without
coating of the dies. In yet another embodiment, the method may employ an
internal release agent in the mat, without coating of the dies.

[0061]Subsequently, the doorskin is allowed to cool (FIG. 2(e)) and then
further processed (sizing and priming) prior to being applied to a
doorframe.

[0062]Thus, the invention describes using a release agent and/or
anti-bonding agent to prevent the thin-layer wood composite from sticking
to the pressing dies during production. In this way, resin levels as high
as 10% to 15% may be used to form doorskins that are only a few
millimeters thick (e.g., about 3 mm), without the composite sticking to
the dies during pressing.

[0063]The release agent and/or anti-bonding agent used to prevent the mat
from sticking to the dies during production may be applied to the mat in
various ways. Generally, when the mat is used to produce a standard
doorskin, one of the dies comprises a recess and is described as the
female die. Referring to FIG. 2, usually the female die 12 is positioned
underneath the lower surface 18 of the mat, which is the surface of the
mat that is adhered to the underlying doorframe (i.e., the inner
surface). The other (upper) surface of the mat 16 corresponds to the side
of the doorskin that will be on the outside of the door. Often, this side
of the doorskin will include a grain texture to improve the decorative
effect. The die 14 used to press the upper side of the mat (i.e. the
eventual outside of the door) may be termed the male die. Thus, the male
die includes a protruding portion that is the mirror image of the recess
on the female die, and optionally, a grain-like pattern on the surface of
the die.

[0064]In one embodiment, an anti-bonding agent is coated onto the bottom
(female) die. Depending on the actual anti-bonding agent used, the
coating may be baked onto the bottom die. In this way, the coated die may
be used several times before recoating with additional anti-bonding
agent. For example, in an embodiment, the step of baking the anti-bonding
agent onto the die surface comprises the steps of: (i) cleaning the die
surface free of any dirt, dust or grease; (ii) spraying about 0.003
inches (3 mils; 0726 mm) of a 50% solution of the anti-bonding agent onto
the die; and (iii) baking the die at over 300° F. (149° C.)
for about 1-4 hours. In an embodiment, the step of cleaning the die
comprises cleaning the die surface with a degreaser; wire brushing to
remove solids; wiping the die surface with a solvent (such as acetone);
and buffing with a cotton pad. The anti-bonding agent is then applied to
provide a 3 mil thickness; and the dies heated to bake the coating onto
the die.

[0065]Under suitable conditions, the anti-bonding agent that is baked onto
the die (or dies) is stable enough to the pressing conditions such that
the die(s) can be used for over 2000 pressing cycles prior to requiring a
second coating with additional anti-bonding agent. Anti-bonding agents
that are suitable for baking onto the die surface include CrystalCoat
MP-313 and Silvue (SDC Coatings, Anaheim, Calif.), ISO-Strip-23 Release
Coating (ICI Polyurethanes, West Deptford, N.J.),
aminoethlyaminopropyltrimethoxysilane (Dow Corning Corporation), or the
like.

[0066]Although a preferred method to facilitate removal of the doorskin
from the die uses a die coated with anti-bonding agent, other equivalent
methods to facilitate non-sticking of the wood composite to the die may
be incorporated into the methods of the present invention. For example,
to facilitate release of the doorskin, the die(s) may be nickel plated,
covered with a ceramic layer, or coated with fluorocarbons.

[0067]As described above, a release agent may be sprayed onto one of the
surfaces of the pre-pressed mat prior to the mat being pressed between
the dies. For example, and referring again to FIG. 2, a release agent 8
may be sprayed onto the upper surface 16 of the mat 6 which is exposed to
the male die 14, Preferably, the release agent 8 sprayed directly onto
the surface of the mat is a release agent that is compatible with the
wood and resin making up the composite. Preferably, the release agent
sprayed on the wood comprises compounds such as PAT®-7299/D2,
PAT®-1667 (Wurtz GmbH & Co., Germany), and the like.

[0068]The amount of release agent sprayed onto at least one side of the
mat may range from 0.1 to 8.0 grams solids per square foot (1.1 to 86.1
grams per square meter) of mat. For example, the release agent may be
sprayed onto the mat as a 25% aqueous solution. In an embodiment, the
amount of release agent sprayed on to at least one side of the mat may
comprise about 4 grams solids per square foot (i.e., 43.05 grams per
square meter) of mat sprayed as a 25% aqueous solution.

[0069]Alternatively, the release agent may be added directly to the
mixture used to form the wood composite. In this embodiment, the release
agent comprises up to about 1 to 8% by weight of the composite. For
example, the release agent may be added as a solution (typically about
25% to 50% solids) and blended with the wood fiber, resin and wax. This
approach has the advantage of not requiring equipment to spray the
release agent onto the mat. Adding the release agent as part of the wood
composite may require the use of more release agent than when only the
surface of the composite is exposed. In some cases (e.g., low production
runs) the cost of the extra materials is justified since the production
set up is simplified.

[0070]The release agent used to coat the mat is distinct from the
anti-bonding agent used to coat the die surface(s). The anti-bonding
agent used to coat the die surface(s) generally may comprise agents such
as silane or silicone based chemicals that are known to be effective
coating agents. These anti-bonding agents, however, are not always
suitable for spraying directly on the wood mat (or incorporating into the
wood composite) since silane or silicone based compounds can interfere
with later finishing of the wood product by priming and/or painting.
Waxes may also act as release agents to some extent. Still, it was found
that waxes common to the door manufacturing industry are generally not
particularly effective in preventing the wood composite from sticking to
either the male or female dies.

[0071]Also, the release agent may be clear, or it may include a pigment.
For example, a tinted release agent comprising the outer surface of a
door would facilitate subsequent priming or painting of the door.

[0072]As described herein, the present invention describes the use of
isocyanate resins to prepare wood composites. One of the advantages of
using isocyanate resins rather than formaldehyde crosslinked resins is
that less energy is needed to dry the wood fiber prior to pressing the
mat. As described herein, traditional phenol-formaldehyde resins are not
compatible with wood having a water content much greater than 8%, as the
water tends to interfere with the curing process. Also, excess moisture
in the wood fiber can cause blistering when pressed with
melamine-formaldehyde resins or urea-formaldehyde resins. Thus, for wood
having a moisture content of greater than 8%, the wood must be dried for
the curing step, and then re-hydrated later. In contrast,
isocyanate-based resins are compatible with wood having a higher water
content and thus, curing with isocyanate-based resins may obviate the
need for the drying and the re-hydrating steps associated with
formaldehyde-based resins.

[0073]To prepare a wood composite that is resistant to water, the
concentration of the isocyanate resin should be at least 5%, and more
preferably be on the order of about 10%. Generally, at levels of about
14-15%, maximum resistance to moisture-induced swelling and/or shrinking
is observed.

[0075]Commercial preparations of the isocyanate resin material may contain
not only 4,4'-methylene diphenyl diisocyanate, but also poly(methylene
diphenyl diisocyanate) otherwise known as polymeric MDI (or PMDI), mixed
methylene diphenyl diisocyanate isomers, and 2,4'-methylene diphenyl
diisocyanate (see e.g., U.S. Pat. No. 6,620,459 for a discussion of the
nature of non-monomeric species in commercial preparations of MDI).
Still, commercially available preparations of 4,4'-methylene diphenyl
diisocyanate give thin-layer composites of high consistency when used as
described herein.

[0076]In an embodiment, the press time and temperature may vary depending
upon the resin used. For example, using a toluene diisocyanate (TDI)
resin as opposed to diphenylmethane diisocyanate (MDI) resin may shorten
the press time by as much as 10%. Generally, when using isocyanate
resins, very high temperatures are not required; thus, isocyanate resins
are associated with decreased energy costs and less wear on the boiler.
Still, composites made at very low temperatures do not display sufficient
resistance to moisture to be commercially useful. Thus, the temperature
used for pressing may range from 250° F. to 400° F.
(121° C. to 204° C.), or more preferably, between
280° F. and 350° F. (138° C. to 177° C.). In
an embodiment, ranges between 310° F. (154° C.) to about
330° F. (166° C.) are preferred.

[0077]The pressure used during pressing may be constant, or varied in a
step-wise fashion. Depending upon the selected temperature and pressure
conditions used for pressing, the total pressing may range from 30
seconds to 5 minutes or more. Thus, the pressure during the pressing step
may include ranges from about 2500 psi (176 kg/cm2) to about 150 psi
(10.5 kg/cm2). Or, the pressure may be applied in a step-wise
manner. For example, the pressure during the pressing step may range from
about 1200 psi (84.3 kg/cm2) for about 5 to 20 seconds followed by
500 psi (35.16 kg/cm2) for 20 to 80 seconds. In one embodiment, the
pressure during the pressure step ranges from about 1200 psi (84.3
kg/cm2) for about 10 seconds to about 500 psi (35.16 kg/cm2)
for about 50 seconds.

[0078]The present invention also encompasses wood products comprising wood
composites made by the method of the invention. For example, in one
aspect, the present invention comprises a wood composite a mixture of:
(a) no more than 95% by weight of a wood fiber, wherein the wood fiber
has a predetermined moisture content; (b) at least 5% by weight of an
organic isocyanate resin; (c) optionally, at least 0.5% by weight of a
wax; (d) optionally, at least 1% by weight of an internal release agent;
and (e) optionally, at least 0.2 grams release agent per square foot
(2.15 grams per square meter) as applied to the surface of the composite.

[0079]Preferably, wood composites made by the method of the invention
comprise significantly less linear expansion and swelling than wood
composites made by conventional methods. Thus, doorskins made by the
method of the present invention exhibit 50% less linear expansion and
thickness swelling than composite doorskins made with formaldehyde-based
resins of the same content (such as, for example, 10%
melamine-urea-formaldehyde doorskins) when boiled in water for 2 hours.
Also, doorskins made by the present invention exhibit 50% less linear
expansion than non-isocyanate based doorskins when immersed in water for
24 hours at 70° F. (21.1° C.), a standard test used in the
industry (ASTM D1037).

[0080]As described above, the thin-layer lignocellulosic composites of the
present invention comprise a predetermined thickness, such that the
resultant composite comprises a flat planar structure. In an embodiment,
the predetermined thickness ranges from 0.100 inches to 0.250 inches
(2.54 mm to 6.35 mm). In an alternate embodiment, the predetermined
thickness of the thin-layer composite may range from 0.110 to 0.130
inches (2.79 to 3.30 mm).

[0081]Also in an embodiment, doorskins made by the methods of the present
invention are significantly less dense than doorskins made using
traditional formaldehyde-based resins. For a doorskin that is 0.12 inches
(3.05 mm) thick and has 10% melamine-urea-formaldehyde resin and 1.5%
wax, the density is about 58 pounds per cubic foot (930 kg/m3). In
contrast, doorskins of the present invention (10% MDI resin; 0.5% wax)
may have a density as low as 50 pounds per cubic foot (801 kg/m3).

EXAMPLE

[0082]Various parameters that would be expected to improve the stability
of doorskins to water were tested, including altering the moisture
content and other attributes of the wood fiber, altering the amount and
type of the resin, and altering the press conditions (temperature,
pressure and/or time).

[0083]Ultimately, it was found that isocyanate-based resin binders
provided a wood composite that is resistant to water when resin levels of
about 10% and up to about 15% were employed. However, when resin at these
levels of resin was used, the resulting composite tended to stick to the
pressing dies during manufacture. For example, in a sample run using 10%
MDI, about 1.5% wax, and 88.5% wood fiber at 10% moisture content,
pressed at a temperature of 320° F. (160° C.) and using
pressing cycles as described herein, it was found that after 6 to 8 press
loads the wood composite would stick to the pressing dies.

[0084]Various methods were tried to prevent the doorskins from sticking to
the dies. It was determined that the addition of a release agent to the
surface of the pre-pressed mat used to make the doorskin allowed the
doorskin to be removed from the male die. In additional experiments, the
release agent was added directly to the composite mixture. For effective
release, approximately 1 to 8% by weight of the release agent was
required. It was found that for consistent results, about 1.5 to 3%
internal release agent was preferred.

[0085]As the release agent is theoretically only required at the surface,
methods to treat the surface of the doorskin were evaluated. It was found
that spraying the surface of the mat with a 25% solution of
PAT®-7299/D2 (Wurtz GmbH & Co., Germany) provided sufficient release
agent to successfully remove the doorskin from the male die. It was
further found that concentrations of release agent ranging from 0.1 to 8
grams solid per square foot (1.1 to 86.1 grams per square meter) of mat
were effective (generally administered as a 25% solution). However, about
2-4 grams release agent solids per square foot (2.2 to 43.05 grams per
square meter) of mat was found to provide consistent results, with higher
concentrations providing only minimally better results.

[0086]Methods were evaluated to apply a release agent to the underside of
the mat and the top surface of the bottom die for each press load. It was
found, however, that treating the surface of the bottom die with an
anti-bonding agent maybe preferable for eliminating bonding of the mat to
the bottom die. An anti-bonding agent, such as Silvue (SDC Coatings) was
used to coat the surface of the female die. Initial experiments used
excess anti-bonding agent to flood the surface of the die. Further
testing indicated that baking the anti-bonding agent onto the surface of
the female (bottom) die allowed for the die to be used multiple times
prior to being retreated. To bake the anti-bonding agent onto the die,
the female die was treated by (i) cleaning the surface of the die free of
dust, dirt and grease using a degreaser, wire brush treatment and
solvent; (ii) spraying about 0.003 inches (3 mils; 0.0762 mm) of a 50%
solution of the release agent onto the die; and (iii) baking the die at a
temperature of about 300° F. (149° C.) to 350° F.
(177° C.) for about 1-4 hours.

[0087]Thus, it was found that addition of 2-4 g per square foot of a
release agent to the upper surface of the pre-pressed mat, and baking the
anti-bonding agent Silvue (SDC Coatings) onto the female (bottom) die
allowed for easy removal of the doorskins having 10% or more MDI resin
from both dies easily. Additionally, it was determined that over 2000
press loads could be made prior to recoating the female die with
additional anti-bonding agent.

[0088]The wood composites made by the method of the invention showed
significantly less linear expansion and swelling than wood composites
made by conventional methods. Thus, doorskins made by the method of the
present invention exhibited 50% less linear expansion and thickness
swelling than composite doorskins made with formaldehyde-based resins of
the same content (e.g., 10% melamine-urea-formaldehyde doorskins) when
boiled in water for 2 hours. Also, doorskins made by the present
invention exhibited 50% less linear expansion than comparable
formaldehyde-based doorskins than non-isocyanate based doorskins when
immersed in water for 24 hours at 70° F. (21.1° C.), a
standard test used in the industry (ASTM D1037).

[0089]Also, doorskins made by the methods of the present invention were
found to be significantly less dense than doorskins made using
traditional formaldehyde-based resins. For example, a doorskin that is
0.12 inches (3.05 mm) thick and has 10% melamine-urea-formaldehyde resin
and 1.5% wax has a density of about 58 pounds per cubic foot (930
kg/m3). In contrast, doorskins of the present invention (10% MDI
resin; 0.5% wax) were found to have a density as low as 50 pounds per
cubic foot (801 kg/m3).

[0090]It will be recognized by those in the art that the advantages of the
methods and compositions disclosed here include:

[0091]1. Preparation of thin-layer lignocellulosic composites, such as
doorskins, that have increased resistance to moisture-induced shrinking
and/or swelling;

[0092]2. Reduced energy costs for preparation of thin-layer
lignocellulosic composites, such as doorskins, in that pre-drying of the
wood is reduced significantly;

[0093]3. A method adaptable to high-throughput production in that multiple
doorskins may be pressed without re-coating of the pressing dies;

[0094]4. Use of isocyanate-based resins at concentrations which provide
high water-resistance in a thin-layer lignocellulosic wood composite; and

[0095]5. Reduced cost of the thin-layer lignocellulosic composite as
additional treatments to impart moisture-resistance are not required.

[0096]It will be understood that each of the elements described above, or
two or more together, may also find utility in applications differing
from the types described. While the invention has been illustrated and
described as a method for high-throughput preparation of thin-layer
lignocellulosic composites, such as doorskins, it is not intended to be
limited to the details shown, since various modifications and
substitutions can be made without departing in any way from the spirit of
the present invention. As such, further modifications and equivalents of
the invention herein disclosed may occur to persons skilled in the art
using no more than routine experimentation, and all such modifications
and equivalents are believed to be within the spirit and scope of the
invention as described herein.